Cutting inserts typically suffer from a limited service life in peripheral rotary milling applications. Peripheral milling (also called “slab milling”) creates a milled surface by using cutting inserts which are located on the periphery of the milling tool. Milling is usually accomplished by rotating the milling tool about a rotation axis parallel to the milled surface of the workpiece. Indexable cutting inserts comprising a positive rake face geometry on both the axial cutting face and the radial cutting face of the insert are commonly employed in operations involving the use of a peripheral rotary tool. An indexable cutting insert includes multiple cutting edges. When a cutting edge that is in use is worn or damaged, an indexable insert can be rotated to assume a different orientation in the insert pocket of a tool holder so that a different cutting edge of the insert is presented to the workpiece. The positive cutting geometry of the inserts reduces the necessary cutting forces and consequently reduces power consumption, resulting in a more efficient cutting operation. Certain cutting inserts used in peripheral rotary milling operations are generally parallelogram-shaped (i.e., each has a generally parallelogram-shaped profile when viewed from a point above the top surface of the insert), with two long sides forming two main cutting edges and two short sides forming two minor cutting edges. These types of cutting inserts provide the capability of a larger depth of cut, although such inserts are not as strong as, for example, square-shaped cutting inserts.
Double-sided cutting inserts double the number of available cutting edges relative to single-sided inserts, thereby creating even greater benefits in cost reduction for both cutting tool end users and cutting tool manufacturers. Presently, double-sided cutting inserts are most commonly used in stationary machining applications, such as turning or grooving, wherein the cutting tool is stationary and the workpiece being machined is rotating. One of the major challenges in developing useful double-sided cutting inserts for a rotary tool is providing for a positive cutting geometry in a rotary machining application, such as hole drilling.
Previously, double-sided cutting inserts were only suitable for use in stationary machining applications. These applications, as noted above, require rotating the piece being machined while the cutting tool remains stationary. In recent years, due to demand for cost reduction and higher productivity from cutting tool end users, double-sided cutting inserts have been developed for milling applications.
Presently available double-sided cutting inserts use an identical cutting profile and cutting geometry for the top and bottom faces. This identical geometry is maintained in order to support an inscribed circle within the insert. It is extremely difficult, if not impossible, to achieve improved machining performance using the same identical cutting insert for a machining application like drilling operations, wherein the cutting speed of a drilling tool varies from zero at the center to a maximum at the periphery. This discrepancy in cutting speeds results because the cutting edge rotates about an axis of rotation to cut the workpiece. The region of the cutting edge that is further from the rotation axis in a radial direction must sweep through a longer path for every revolution about the axis. Regions of the cutting edge radially closer to the rotational axis, conversely, sweep a shorter distance per revolution. Any portion of a cutting edge at the axis remains in the same position during rotation about the axis and, thus, does not sweep any distance per revolution of the tool. Therefore, the cutting speed (i.e., the distance swept by the cutting edge per time) varies from zero at the axis of rotation to a maximum at the cutting edge region radially furthest from the axis. As cutting speed increases, the cutting power on the cutting edge increases, resulting in greater wear.
Thus, there is a need for an improved double-sided cutting insert that overcomes the drilling performance limitations resulting from variation in cutting speeds along the cutting edge in the radial direction, as well as other disadvantages and shortcomings.